1. Field of the Invention
The present invention relates to a photoresist exposure apparatus for forming a pattern on the surface of a printed-wiring board having through holes using a photolithographic technology, and more particularly to a photoresist exposure apparatus for inducing a proper photochemical reaction in a photosensitive resin (electrodeposited photoresist) which is electrically deposited on the surface of the board and on the inner wall surfaces of the through-holes to thereby improve a working efficiency for an exposure process.
2. Description of the Related Art
In a conventional pattern-forming process in which using the photolithographic technology, patterns are formed on both sides of a printed-wiring board having through-holes therein and then electrically connected through a copper-plated layer which is coated on the inner wall surface of each of the through-holes, the following treatments have been made in order to prevent the copper-plated layer from being damaged due to an etching treatment: (1) the surface of the board is covered with a dry film, and the opening of each through-hole is closed with a thin resist film 1 which has been hardened by irradiation of light (tenting method) as shown in FIG. 15, or (2) after exposure and development processes, solder 4 is plated over the exposed surface of the copper-plated layer (solder/through-hole method). Therefore, the pretreatment for an etching process is very complicate. In FIGS. 15 and 16, reference numerals 5, 6 and 7 denote a board (copper-clad laminate), an insulating substrate and a copper foil, respectively, and a reference numeral 9 (a part sectioned by dotted lines) denotes parts to be removed in the etching process.
In order to overcome the disadvantage as described above, the same applicant as the present application has proposed an exposure apparatus in which an electrodeposition-type photoresist 10 (a photosensitive resin having electrodepositing capability) is coated on the surface of the board and on the inner wall surfaces of the through-holes as shown in FIG. 17, and ultraviolet rays used in a pattern forming process are irradiated to the through-holes 2 to harden the photoresist 10, thereby protecting the copper-plated layer 3 (as disclosed in Japanese Laid-Open (Unexamined Published) Patent Application No. HEI-2-254455).
The exposure apparatus includes a light source 14 having plural ultraviolet lamps 13 and foldable plane reflection mirrors 15, the light source 14 and the reflection mirrors 15 being disposed so as to be freely rotatably in both clockwise and counterclockwise directions arouund axes 16 and 17 in parallel to the board 5, respectively, as shown in FIG. 18. In the exposure apparatus thus constructed, the light source 14 is horizontally disposed adjacently to the board 5 so as to sandwich the board 5 as indicated by a solid line, and in this state the ultraviolet rays are irradiated to the board 5 to photocure the photoresist on the inner walls of the through-holes. Thereafter, the light source 14 is vertically disposed to the board 5 as indicated by an imaginary line (two-dotted line), and in this state the board 5 is irradiated with the ultraviolet rays through plane reflection mirrors 15 which are slantly disposed to the board 5 as indicated by an imaginary line (two-dotted line) of FIG. 18 to photocure the photoresist on the surface of the board.
As described above, this apparatus performs the protection of the copper-plated layer in the through holes with the photocured photoresist, so that the copper-plated layer coated on the inner wall surface of the through-holes can be prevented from being damaged without onerous pretreatments as described above. In addition, the light source 14 is used to satisfy two requirements, that is, serves not only to form a pattern on the board 5, but also to protect the through-holes, so that the exposure process can be carried out with only one exposure apparatus.
However, there are still some improvements as described below which should be made to the aforementioned exposure apparatus.
When the exposure treatment is conducted on the inside of the through-holes, the ultraviolet rays 18 are irradiated into the through-holes at an inclination angle which is vertical or nearly vertical to the board 5, the intensity of the exposure light (ultraviolet rays) for the inside of the through-holes is more extremely great than that of the exposure light for the surface of the board 5. This reason will be described later.
Accordingly, when an exposure process is carried out for the inside of the through-holes, the photoresist 10 coated on the surface of the board must be covered by a mask (not shown). On the other hand, when another exposure is carred out for the surface of the board, the mask must be first removed, and then the exposure process is conducted through a pattern-forming film. Therefore, the exposure process can not be smoothly carried out.
The relationship between an incident angle of the ultraviolet rays and the intensity of the ultraviolet rays to photocure the photoresist will be hereunder described.
As shown in FIG. 17, when the ultraviolet rays are irradiated at an inclination angle .alpha. to the board 5 which is coated with the photoresist 10 and has a thickness of d, an exposure amount of the ultraviolet rays required for photocuring the photoresist 10 having a thicknessof d1 which is determined by the equation (1), and another exposure amount of the ultraviolet rays required for photocuring the photoresist 10 having a thickness of d2 which is provided by the equation (2) are required for the exposure of the inside of the through holes and the surface of the board, respectively. Therefore, the ratio of both exposure amounts (d1/d2) is equal to tan.alpha.. EQU d1=d/cos .alpha. (1) EQU d2=d/sin .alpha. (2)
The ratio of the exposure amounts of the ultraviolet rays, tan.alpha., for example, d1/d2 is approximately equal to 11 for .alpha.=85 degrees, and thus a stronger ultraviolet light is needed for the exposure of the photoresist on the inner wall surfaces of the through-holes. On the other hand, the ration d1/d2 is approximately equal to 2.74 for .alpha.=70. From these data, it is clearly understood that the difference in inclination angle causes the remarkably variation in the value of d1/d2.
In addition, the exposure apparatus shown in FIG. 18 needs plural ultraviolet lamps to irradiate the ultraviolet rays to the through-holes at an edge portion of the board, so that a demanded power for the light source is increased.
In order to solve the aforementioned problems, an exposure apparatus in which the ultraviolet rays are irradiated to the overall board 5 to simultaneously photocure the electrodeposited photoresists coated on the inner wall surfaces of the through-holes and on the surface of the board has been intensively researched. This apparatus includes a single electric discharge lamp 13, a reflection mirror 20 which constitute upper and lower ultraviolet rays irradiating sources, and light-transmittable members 22 such as a ground glass which are vertically symmetrically disposed at middle portions between the ultraviolet rays irradiating sources and the board 5. The ultraviolet rays emitted from the ultraviolet rays irradiating sources are passed through the light-transmittable members 22 to be scattered, so that the substantially homogeneous lights 23 having no directivity can be irradiated onto the overall surface of the board 5.
Nevertheless, this kind of apparatus had to be given up according to a lot of research results for the reasons below:
(1) The light transmitted through the light-transmittable members 22 is weaker in intensity than an expected value, and thus it takes a long time to photocure the resist on the inner walls of the through-holes, so that the photoresist may be peeled off due to a thermal action of infrared ray contained in the irradiated light or may be lost in a developing process due to incomplete photocuring of the photoresist.
(2) When in order to perform the proper exposure for the photoresist inside the through-holes, an output of the electric discharging lamp 13 is increased, the photoresist on the surface of the board is excessively photocured and it is difficult to remove the resist in the developing process and after the etching process.